Vacancies: -vacant atomic sites in a structure. Self-Interstitials: -"extra" atoms positioned between atomic sites. POINT DEFECTS CHAPTER 4: IMPERFECTIONS.

• Vacancies:-vacant atomic sites in a structure.

Vacancydistortion of planes

• Self-Interstitials:-"extra" atoms positioned between atomic sites.

self-interstitialdistortion

of planes

POINT DEFECTS

CHAPTER 4:IMPERFECTIONS IN SOLIDS

Two outcomes if impurity (B) added to host (A):• Solid solution of B in A (i.e., random dist. of point defects)

• Solid solution of B in A plus particles of a new phase (usually for a larger amount of B)

OR

Substitutional alloy(e.g., Cu in Ni)

Interstitial alloy

(e.g., C in Fe)

Second phase particle--different composition--often different structure.

POINT DEFECTS IN ALLOYS

• Low energy electron microscope view of a (111) surface of Cu.• Sn islands move along the surface and "alloy" the Cu with Sn atoms, to make "bronze".• The islands continually move into "unalloyed" regions and leave tiny bronze particles in their wake.• Eventually, the islands disappear.

ALLOYING A SURFACE

• Dislocations slip planes incrementally...• The dislocation line (the moving red dot)... ...separates slipped material on the left from unslipped material on the right.

INCREMENTAL SLIP

LINE DEFECTS

Atomic view of edgedislocation motion fromleft to right as a crystalis sheared.

DISLOCATIONS

EDGE DISLOCATIONS

SCREW DISLOCATIONS

Grain boundaries: • are boundaries between crystals. • are produced by the solidification process, for example. • have a change in crystal orientation across them. • impede dislocation motion.

grain boundaries

Schematic

heat flow

~ 8cmMetal Ingot

AREA DEFECTS: GRAIN BOUNDARIES

Fe-Cr alloy

microscope

grain boundarysurface groove

polished surface

Grain boundaries...• are imperfections,• are more susceptible to etching,• may be revealed as dark lines,• change direction in a polycrystal.

ASTM grain size number

N = 2n-1

no. grains/in2 at 100x magnification

OPTICAL MICROSCOPY

RESOLUTION RANGES